Document --homefile behaviour

[shilorigins]

What's the problem this feature will solve?

Setting --homefile from the CLI or the home_file parameter in PyDMApplication loads the chosen file into a display during startup. Clicking the home button in the navigation bar then simply shows this pre-loaded display.

This behaviour has significant performance impacts (~35% of LCLSHome's startup time is dedicated to loading the homefile) and can be unintuitive. For example, clicking the home button has no effect if the home display is already being shown, which can be confusing if the home display is stateful. Additionally, if the homefile is the same as the file being launched, two instances of the same display are created.

Describe the solution you'd like

I'd like this behaviour to be documented so that developers are aware of the potential performance impacts and actual behaviour of PyDM's built-in home button.

Additional context